1. Field of the Invention
The invention relates generally to inorganic chemistry and specifically to methods for producing high-grade alumina for catalytic applications.
2. Description of the Prior Art
Aluminum oxide (alumina) occurs abundantly in nature, most often as impure hydroxides, e.g., as in bauxites and laterites. Major chemical products are made by purifying such natural ores. Most bauxite is refined by the Bayer process which uses caustic additions to remove impurities and to produce a nominal 99.5% Al.sub.2 O.sub.3 product with Na.sub.2 O as its dominant impurity. About 85% of alumina is used in the production of aluminum metal. The rest is consumed in other applications, including activated aluminas.
Activated aluminas are widely used in adsorption and catalysis where their relatively large surface areas, pore structure and surface chemistry play important roles. Hydrated aluminas, those with water, are dehydrated by controlled heating. The oldest commercial form, still in wide use, is made from Bayer alpha-trihydrate. Activated bauxites have similar properties to the activated alumina from the Bayer alpha-trihydrate. They are obtained by thermal activation of bauxite containing alumina in the form of gibbsite. Another type of activated alumina is obtained by very rapid activation of Bayer hydrate at 673.degree.-1073.degree. K. The outcome is essentially amorphous alumina with a weak pattern of .gamma./.eta.-alumina. Alumina gels also serve as starting points for the manufacture of activated aluminas. These gels are prepared from solutions of Al.sub.2 (SO.sub.4).sub.3 and produce corresponding by-product salts that precipitate out after being washed.
Activated aluminas find important applications as catalysts. Sometimes the Na.sub.2 O content is reduced to under one-tenth of one percent by washing in acidified water. Preparations using refined Al or an alkoxide of Al can be used to make an extremely pure activated alumina gel. But the manufacturing costs of these high-purity aluminas are very high.
The catalytic reactivity of activated alumina is represented by its theoretical number of available active sites. The surfaces contain hydroxyl groups, oxides and aluminum ions. The three basic catalytic sites also have many possible logistical combinations.
A major catalytic application of activated alumina is that of Claus converters, which recover sulfur (S) from H.sub.2 S that has been extracted from sour natural gas or refinery off-gas. The dehydration of alcohol is one of the oldest catalytic reactions. Activated alumina can initiate synthesis in which water may be the reactant or the product. Bulk MoO.sub.3 is industrially reduced to metal at 773.degree. K. with H.sub.2, but when supported on activated alumina, the reaction proceeds only to MoO.sub.2. One of the largest modern-day uses of activated alumina is that of a catalyst support for catalytic mufflers on automobiles. The catalyst is a blend of Pt and Pd metals supported on pellets or a monolithic form. The Pt-Pd is used as an oxidation catalyst to convert hydrocarbons and CO to CO.sub.2 and H.sub.2 O.
Poisons to the active sites of activated alumina catalysts were thought by most to comprise the naturally occurring impurities of potassium (K) and sodium (Na). The present inventors have discovered that potassium (K) is not necessarily a catalytic poison, especially when present in only small residual quantities.